1. Field of the Invention
This invention relates to optical amplifiers and more particularly to an optical amplifier that uses stimulated Brillouin scattering for providing optical amplification.
2. Description of the Related Art
To this point, optical signal amplifiers have been dominated by Raman amplifiers. For example, U.S. Pat. No. 3,414,354, issued to E. H. Siegler Jr., entitled Raman Spectrometers, is a seminal publication disclosing the use of stimulated Raman scattering, to provide optical amplification. In a later example, U.S. Pat. No. 3,515,897, issued to W. H. Culver, discloses a design for implementing stimulated Raman scattering as an amplifier.
Use of Raman scattering; however, has limitations in its operation and implementation. Examination of the equations that govern stimulated Raman scattering break down into two terms. The first term is associated with the wave that is being amplified, also known as the Stokes wave. The second term is associated with a material excitation that is a product of the Raman scattering. That material excitation causes inherent inefficiencies and engineering difficulties that cannot be removed. Consequently, stimulated Raman scattering can be considered as a parametric or coupled generation process in which the optical pump wave generates a Stokes wave (i.e. the amplified input) and a material excitation wave. This material excitation wave is part of the coupled wave physical process which allows the input beam to be amplified, but does not contribute anything to the desired amplification. The energy that is distributed to the material excitation is lost to the optical output. Furthermore, this material wave eventually couples its energy into thermal excitations within the media, so that it contributes to waste heat in the process. This heat can lead to immediate distortions in the efficiency of the optical amplifier and long-term deterioration of the amplifier medium itself. Considerable engineering must take place in the optical design to handle this problem, causing the system to be bulkier and heavier than it might be otherwise.
The inherent difficulty with Raman scattering is that the material excitation itself is a high energy excitation. In order to use a Raman active medium for amplification, the optical implementation is constrained to excite the material parameter inherent to said medium. These excitations are associated with vibrational resonances in the infrared segment of the electromagnetic spectrum. The associated wavelengths of these excitations will be in the 3 to 10 micron regime. A typical amplifier beam will be in the mid-visible, at a wavelength of approximately 0.5 micron. Consequently, 10% of the pump beam will be lost to the material excitation, even if the optical system is lossless otherwise. For high power long term operation, this is a considerable loss.